1. Field of the Invention
The present invention relates to a catalyst composition and an exhaust gas purifying method. In particular, the present invention relates to an exhaust gas purifying catalyst and an exhaust gas purifying method for purifying an exhaust gas from an internal combustion engine of an automobile and the like.
2. Description of Related Art
Nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HC) and so on are contained in the exhaust gas from an internal combustion engine such as an automobile engine. Accordingly, generally, after being purified with an exhaust gas purifying catalyst that oxidizes CO and HC and reduces NOx, the exhaust gas is discharged into air.
Further, the exhaust gas that is discharged from the internal combustion engine contains, in addition to the nitrogen oxides and other above-mentioned substances, particulate materials referred to as particulate matter (PM) or particulate. In recent years, the restriction on the PM has become more severe than ever, and accordingly, it is demanded to efficiently remove the PM together with the nitrogen oxides and other substances.
For removing the PM, a method in which the PM is collected on a filter such as a diesel particulate filter, and the collected PM is removed by oxidation by intermittently raising a temperature of the filter.
However, a high temperature is necessary to remove the PM collected on the filter by oxidation, and this may cause degradation of the fuel consumption. Further, in the PM removal that depends on the filter, pressure loss due to the filter may also cause degradation of the fuel consumption.
For such a problem, a method is proposed in which by use of a catalyst having strong oxidizing power, the oxidation removal of PMs is promoted.
Regarding this, Japanese Patent Application Publication No. 2010-196552 (JP 2010-196552 A) proposes a method in which in exhaust gas purification for an internal combustion engine that performs a theoretical air-fuel ratio (stoichiometric) operation, a material having ceria (Ce2O3) or praseodymium oxide (Pr2O3) as a main component and a material having neodymium oxide (Nd2O3) as a main component are used and, as required, an air-fuel ratio of the exhaust gas is slightly and continuously fluctuated around the theoretical air-fuel ratio (14.5), more specifically, for example, the air-fuel ratio is continuously fluctuated in the range of 14.0 to 15.0, thus, ceria or praseodymium oxide is oxidized and reduced and active oxygen is generated thereby, and the PMs in the exhaust gas are removed by oxidation with the active oxygen.
The present invention provides an exhaust gas purifying catalyst and an exhaust gas purifying method for removing the PMs by oxidation.
JP 2010-196552 A proposes a method for removing the PMs by oxidation in an internal combustion engine that performs a theoretical air-fuel ratio (stoichiometric) operation. However, according to the method, since purification of the exhaust gas from an internal combustion engine that performs the theoretical air-fuel ratio (stoichiometric) operation is supposed, there was a room for improvement in the fuel consumption.